This is the "Pairs with Positive Negative values" problem from geeksforgeeks.org.

Given an array of distinct integers, print all the pairs having positive value and negative value of a number that exists in the array.

NOTE: If there is no such pair in the array , print "0".

The first line consists of an integer T i.e number of test cases. The first line of each test case consists of an integer n. The next line of each test case consists of n spaced integers.

Print all the required pairs in the increasing order of their absolute numbers.

Here is my code:

def findPairs(l):
    for i in range(listLen):
        if (-1*l[i]) in l[i+1:]:
    #print (nums)
    for num in nums:

    return pairs    

for i in range(T):
    if len(pair):
        print(" ".join(str(x) for x in pair))

1 Answer 1


You definitely incorporated the response to your previous questions, such as:

  • Extract code into functions.
  • Always indent with 4 spaces.
  • Process input immediately instead of storing all input data and results in lists.

There is still too little (horizontal) whitespace, I suggest to check your code against the PEP8 coding style, for example at PEP8 online.

Also function and variable names should be snake_case according to PEP8.

More suggestions:

  • Variable names: T is too short and non-descriptive, num_tests would be better. pair is better named pairs.
  • There is no need to pre-assign pair=[].
  • The value of the iterator variable i in the main loop is not needed, a common convention is use _ instead:

    for _ in range(T):
  • Negation as in -1*l[i] can be simplified to -l[i]

  • When printing the result list

    print(" ".join(str(x) for x in pair))

    you can use map instead, as you already do when reading the input:

    print(" ".join(map(str, pair)))

Improving the performance:

Your algorithm has two nested loops traversing through the list, i.e. the time complexity is \$ O(n^2) \$ as a function of the list length.

This can be improved by sorting the list first. One option would be to sort the numbers in increasing order, e.g.

-3 -1 1 2 3 6

for the first test case, and then traverse the list from both ends to find pairs. The time complexity is \$ O(n \log n) \$ for the sorting, and \$ O(n) \$ for the traversal, so this would be faster for large input.

Another option is to sort the number in increasing absolute value, e.g.

 -3 3 -1 1 2 6

Again a linear sweep of the sorted list is now sufficient to find matching pairs, which are now consecutive entries.

Here is a possible implementation of the second approach:

def find_pairs(numbers):
    numbers.sort(key=lambda x: x + 0.5 if x >= 0 else -x)
    pairs = []
    for pair in [(x, y) for x, y in zip(numbers, numbers[1:]) if x == -y]:
    return pairs


  • sort() with a custom key. This relies on the given fact that the numbers are distinct integers.
  • zip and list comprehension to enumerate pairs of consecutive list entries.

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